I always felt that Mycroft reminding Sherlock about Redbeard was his way of telling him, "Remember that time you loved something and it was taken from you? Don't let it happen again." Mycroft knows that Sherlock loves Mary and especially John, and is telling him not to get involved because they will be focusing more on each other now with no time for Sherlock.

Thank you and no I use mark pen，I never had a dog more than one month，I know how soft and sweet they are but I have no chance to be with them until the day come……maybe someday in further I will have a dog，my dog，I think that would be a great thing even one day he will leave，because the most important thing is not the day we lost them，but the days them with us，right？